TOCTOU: vq_write_ok check and vfs_stdaccount commit are separate critical sections

Summary

vq_write_ok takes ac_spin, checks limit, drops lock(:483). VFS allocates blocks. vfs_stdaccount re-takes ac_spin and updates(:158). N concurrent writers can each pass check and collectively exceed limit by (N-1)*delta. Advisory limits under concurrency. Gated by vfs_quota_enabled.